1. Field of the Invention
The present invention relates to a masterbatch having an excellent processability for producing an oxygen-absorbing molded article having an excellent oxygen-barrier property, and relates to a production method of the oxygen-absorbing molded article made of the masterbatch.
2. Description of the Prior Art
Conventionally, metallic cans and glass bottles have been used as packaging containers capable of inhibiting oxygen from entering thereinto from outside and exhibiting a high ability to store the contents, but have been recently increasingly replaced with plastic packaging containers made of an oxygen-barrier thermoplastic resin in view of its good processability and low costs. As the oxygen-barrier thermoplastic resins, ethylene-vinyl alcohol copolymers and polyamides produced by the polycondensation of a diamine component mainly composed of m-xylylenediamine and a dicarboxylic acid component mainly composed of adipic acid (hereinafter referred to as “nylon MXD6”) have been widely used because of their low permeability to gaseous substances such as oxygen and carbon dioxide gas, good processability, sufficient transparency and sufficient mechanical strength. The packaging containers made of metal or glass are substantially free from the permeation of gas into its inside from the outside. In contrast, a non-negligible amount of gas permeates into the inside from the outside in the packaging containers made of the oxygen-barrier thermoplastic resins. The amount of permeated gas increases in some cases according to the conditions for storing the packaging containers made of the oxygen-barrier thermoplastic resin. Therefore, the packaging containers made of the oxygen-barrier thermoplastic resin are poor in the ability to store the contents for a long time, as compared to the conventional metallic cans and glass bottles.
Recently, compositions prepared by adding a cobalt compound to an unsaturated carbon-carbon bond-containing thermoplastic resin such as polybutadiene and polyisoprene or nylon MXD6, compositions prepared by adding a cobalt compound to a mixture of polyamide, polyethylene terephthalate, etc., with an unsaturated carbon-carbon bond-containing thermoplastic resin such as polybutadiene and polyisoprene, and compositions prepared by adding a cobalt compound to a modified resin obtained by introducing an unsaturated carbon-carbon bond to molecular chains of polyethylene terephthalate have been found to exhibit an oxygen absorbing function, and packaging containers using such oxygen-absorbing compositions have now been put into practice. In these packaging containers, since the permeating oxygen from the outside is absorbed by the oxygen-absorbing thermoplastic resin, the amount of oxygen permeating from the outside is apparently considerably reduced. In addition, the oxygen present in the packaging containers is also absorbed by the oxygen-absorbing thermoplastic resin, thereby preventing oxidative deterioration of the contents over a long period of time, and enhancing the ability to preserve the contents as compared to the conventional packaging containers.
There have been conventionally proposed various methods for producing the above oxygen-absorbing thermoplastic resin compositions by adding a transition metal compound to a thermoplastic resin. For example, there are known methods of producing oxygen-absorbing resin compositions by mixing thermoplastic resin pellets containing nylon MXD6 as an oxidizable thermoplastic resin with a solution of a transition metal compound, if required under reflux, and then evaporating the solvent to allow the transition metal compound to adhere to the thermoplastic resin pellets (Japanese Patent Application Laid-Open Nos. 2-500846 and 3-505888). However, the proposed methods require the drying step for evaporating the solvent which entails some danger. Therefore, a special drying apparatus for evaporating the solvent is needed. In addition, it takes a very long time until the drying step is completed, increasing the production costs. In the proposed methods, since the thermoplastic resin pellets carrying the transition metal compound is dried by long-term heating, the oxidation of the nylon MXD6 by the transition metal compound is promoted during the heat-drying, thereby likely to cause the reduction of molecular weight of the nylon MXD6 and the decrease of melt viscosity of the resultant oxygen-absorbing resin composition. This tends to adversely affect various subsequent molding steps to cause molding defects, etc.
Alternatively, there have been proposed a method of producing oxygen-absorbing resin compositions in which a mixture of thermoplastic resin pellets containing nylon MXD6 and a transition metal compound is melt-kneaded in an extruder, etc., extruded into strands, and then pelletized (Japanese Patent Application Laid-Open No. 11-514385). However, in the proposed method, since the molten thermoplastic resin containing nylon MXD6 is kneaded with the metal compound, the oxidation of nylon MXD6 is promoted during the kneading to reduce the molecular weight, thereby likely to reduced the melt viscosity of the resultant oxygen-absorbing resin composition. In some cases, the reduction of molecular weight of the nylon MXD6 may be more significant according to the melt-kneading conditions to lead to a drastic decrease of the melt viscosity of the oxygen-absorbing resin composition. When such an oxygen-absorbing resin composition is subjected to the subsequent molding steps, there tend to occur problems such as molding defect because of uneven melt viscosity.
Not limited to the above methods, in the methods for producing molded articles from the oxygen-absorbing resin composition obtained by melt-mixing an oxidizable thermoplastic resin with a transition metal compound, the oxidizable thermoplastic resin inevitably suffers from oxidative deterioration upon molding, thereby adversely affecting the moldability into products. Also, when allowed to stand in an oxygen-containing atmosphere such as in air, the molecular weight of the oxidizable thermoplastic resin is continuously reduced because of the oxygen absorption by the oxygen-absorbing resin composition. Therefore, the oxygen-absorbing resin composition should be stored until it is used for molding so as to prevent it from coming into contact with oxygen as effectively as possible.